Nor, finally, can the federalization of registration failures be justified by the two chief policy concerns often warranting federal involvement: externalities and interjurisdictional challenges. Unlike firearms regulation, for instance, no evidence exists of state registration laxness contributing to spillover harms in other states. Likewise, interstate travel creates no enforcement barrier, such as prompted prior federal efforts to criminalize "deadbeat dads" whose travel confounded state prosecution. Persons such as Madera can be, and traditionally have been, readily prosecuted under state law.

Frustration over how to handle emigrant sex offenders affords a highly compelling catalyst for increased federal involvement in criminal justice. The challenge, however, should not be resolved by the coming court battles over congressional authority alone, but also with due regard for the impact upon the nation's governing architecture and political culture as a whole.